Publication Date:
2002
abstract:
We have analysed SCUBA 850 mu m images of the (near) face-on spiral galaxy
NGC 6946, paying particular attention to the subtraction of sky signal. A
comparison with both 21 cm HI and 12CO(2-1) intensity maps reveals a tight
correlation between dust thermal emission and molecular gas at the
kiloparsec level. By means of a Monte Carlo radiative transfer model, we
convert a B-K colour image of NGC 6946 into a map of visual optical depth.
The model yields maximum opacities since we assume that any increase in B-K
colour, with respect to the disk edge, is attributable solely to extinction
by dust. The resultant map of visual optical depth relates well to the
distribution of neutral gas (HI+H2) and implies a global gas-to-dust ratio
of 90 (this value is a lower limit). There is no significant radial
variation of this ratio: this can be understood, since the gas content is
dominated by far by the molecular gas. The latter is estimated through the
CO emission tracer, which is itself dependent on metallicity, similarly to
dust emission. In the absence of a more objective tracer, it is not
possible to derive the true gas-to-dust ratio. By comparing the radial
profile of our visual optical depth map with that of the SCUBA image, we
infer an emissivity (dust absorption coefficient) at 850 mu m that is 3
times lower than the value measured by COBE in the Milky Way, and 9 times
lower than in NGC 891. We view this very much as a lower estimate, however,
given our initial assumptions in deriving the visual opacity, and the
possibility of underestimating the large-scale submm emission, the effect
being more severe for the nearly face-on orientation of NGC 6946. A
decomposition of the spiral structure half way out along the disk of NGC
6946 suggests an interarm optical depth of between 1 and 2. These
surprisingly high values represent 40-80% of the visual opacity that we
measure for the arm region.
NGC 6946, paying particular attention to the subtraction of sky signal. A
comparison with both 21 cm HI and 12CO(2-1) intensity maps reveals a tight
correlation between dust thermal emission and molecular gas at the
kiloparsec level. By means of a Monte Carlo radiative transfer model, we
convert a B-K colour image of NGC 6946 into a map of visual optical depth.
The model yields maximum opacities since we assume that any increase in B-K
colour, with respect to the disk edge, is attributable solely to extinction
by dust. The resultant map of visual optical depth relates well to the
distribution of neutral gas (HI+H2) and implies a global gas-to-dust ratio
of 90 (this value is a lower limit). There is no significant radial
variation of this ratio: this can be understood, since the gas content is
dominated by far by the molecular gas. The latter is estimated through the
CO emission tracer, which is itself dependent on metallicity, similarly to
dust emission. In the absence of a more objective tracer, it is not
possible to derive the true gas-to-dust ratio. By comparing the radial
profile of our visual optical depth map with that of the SCUBA image, we
infer an emissivity (dust absorption coefficient) at 850 mu m that is 3
times lower than the value measured by COBE in the Milky Way, and 9 times
lower than in NGC 891. We view this very much as a lower estimate, however,
given our initial assumptions in deriving the visual opacity, and the
possibility of underestimating the large-scale submm emission, the effect
being more severe for the nearly face-on orientation of NGC 6946. A
decomposition of the spiral structure half way out along the disk of NGC
6946 suggests an interarm optical depth of between 1 and 2. These
surprisingly high values represent 40-80% of the visual opacity that we
measure for the arm region.
Iris type:
01.01 Articolo in rivista